1. Field of the Invention
This invention broadly relates to an appliance used in the course of orthodontic treatment. More particularly, the present invention relates to a ceramic orthodontic appliance having an archwire slot liner for receiving an archwire.
2. Description of the Related Art
Orthodontic treatment is directed to movement of the teeth to improved positions. Orthodontic treatment can greatly enhance the patient's facial appearance, especially in areas near the front of the patient's mouth. Orthodontic treatment can also improve the patient's occlusion so that the teeth function better with each other during mastication.
One type of orthodontic treatment involves the use of a set of appliances and archwires that are commonly known collectively as “braces”. During treatment, tiny, slotted appliances known as brackets are affixed to the patient's anterior, cuspid and bicuspid teeth, and an archwire is placed in the slot of each bracket. The archwire forms a track to guide movement of the teeth to orthodontically correct positions. Ends of the archwire are often received in passages of small appliances known as buccal tubes that are affixed to the patient's molar teeth.
Orthodontic appliances are widely available in a variety of materials. Many orthodontists prefer to use appliances made of a metallic material such as stainless steel because metal appliances slide along metal archwires with relatively little resistance from the forces of friction. Unfortunately, metal appliances are often visible in the mouth and considered unaesthetic by many. The use of metal brackets sometimes leads to comments of a “metallic mouth appearance” that can be an embarrassment to the patient.
Some orthodontic appliances are made of a plastic material having a neutral color or a color that matches the color of the teeth. When new, plastic appliances are generally considered more aesthetic than metallic appliances. Unfortunately, some plastic appliances are stained by certain food and beverages and turn an unsightly color after a period of time. Moreover, the plastic material may slowly creep in use to such an extent that the archwire slot widens and precise control over movement of the tooth is rendered difficult.
Orthodontic appliances that are made of a transparent or translucent ceramic material overcome many of the problems associated with metal and plastic appliances. U.S. Pat. No. 4,954,080, assigned to the assignee of the present invention, describes a color-free ceramic bracket made of a polycrystalline material with a translucency that permits the natural color of the tooth to diffusely show through the bracket. Additionally, ceramic material is resistant to staining and does not deform by creep as in the case with plastic appliances.
Examples of orthodontic appliances made of a ceramic material are described in U.S. Pat. Nos. 5,358,402, 5,366,372, 5,380,196 and 5,439,379. The ceramic appliances that are described in those references have a metallic archwire slot liner that is located in a channel of the appliance. During treatment, the archwire is placed in the archwire slot liner and consequently a metal-to-metal contact between the archwire and the appliance is provided. Many practitioners prefer to use ceramic appliances with metallic archwire slot liners because they believe that the metal-to-metal contact between the appliances and the archwire provides better sliding mechanics than is typically observed with the use of ceramic appliances that lack metallic archwire slot liners.
Presently, there is a continuing interest in reducing the size of orthodontic appliances. Smaller appliances are more difficult to see in use and as a result are often considered more aesthetic. Although ceramic orthodontic appliances made of a translucent, colorless material are widely considered aesthetic, there is a continuing desire to reduce the size of such appliances to further enhance the patient's appearance during treatment.
Moreover, in some instances the appliances may come into contact with soft tissue, opposing dentition or other orthodontic appliances in the patient's oral cavity during the course of treatment. Such contact can lead to discomfort and occasionally pain that is best avoided if at all possible. Reducing the size of orthodontic appliances is a benefit, in that the probability of such contact is decreased.
Over the years, many attempts have been made to reduce the overall size of orthodontic appliances. Unfortunately, the reduction in appliance size may also reduce the fracture strength of the appliance to an amount that is deemed unacceptable. If an appliance fractures during the course of treatment, the patient should return to the practitioner for replacement of the appliance so that treatment can resume. As can be appreciated, appliance fracture represents a nuisance to both the practitioner and the patient in terms of time and expense.